Most states have long recognized a form of the insanity defense, based on the defendant’s mental illness, defect, or inability to understand that the criminal act was wrong. In pleading an insanity defense, the defendant admits the criminal conduct, but asserts a lack of culpability based on mental illness. Many states still model their insanity defense on the old English rule of law (the M’Naghten rule from 1843) in which the defendant asserts he (1) did not know the nature and quality of the act, or (2) did not know that it was wrong.

And it is an affirmative defense to a prosecution under any federal statute (federal law) that, at the time of the offense, the defendant was unable to appreciate the nature and quality of his acts, or the wrongfulness of his acts. See 18 U.S.C. §17.

When a defendant is found not guilty by reason of insanity it does not mean he necessarily goes free. States often have requirements for treatment or institutionalization after such a finding. And some states require, at a minimum, confinement in a treatment institution or facility for the length of time the person would have received if convicted—so a defendant may end up spending more time confined than if he did not raise such a defense.

The law regarding the availability, definitions, and nature of the insanity defense vary from state to state, and are usually located in a state’s statutes.

In Texas, the insanity defense is recognized and allows a defendant to claim they were legally insane at the time of committing a crime, thus lacking culpability due to mental illness. Texas follows a variation of the M'Naghten Rule, which requires the defendant to prove that, as a result of severe mental disease or defect, they did not know their conduct was wrong. The burden of proof is on the defense, and the standard is 'clear and convincing evidence.' If a defendant is found not guilty by reason of insanity, they are not automatically set free. Instead, Texas law mandates that the individual be committed to a mental health facility for treatment. The duration of confinement can be equivalent to, or potentially longer than, the prison sentence that would have been imposed had the defendant been convicted of the crime. The specifics of the insanity defense in Texas are codified in the Texas Penal Code, and federal law also provides for an insanity defense under 18 U.S.C. §17, which has a similar standard to the state law but is applied in federal prosecutions.